1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of surface related multiple prediction in marine seismic surveys.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey.
The resulting seismic data obtained in performing a seismic survey is processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed. The processed seismic data is processed for display and analysis of potential hydrocarbon content of these subterranean formations. The goal of seismic data processing is to extract from the seismic data as much information as possible regarding the subterranean formations in order to adequately image the geologic subsurface. In order to identify locations in the Earth's subsurface where there is a probability for finding petroleum accumulations, large sums of money are expended in gathering, processing, and interpreting seismic data. The process of constructing the reflector surfaces defining the subterranean earth layers of interest from the recorded seismic data provides an image of the earth in depth or time. The image of the structure of the Earth's subsurface is produced in order to enable an interpreter to select locations with the greatest probability of having petroleum accumulations.
In a marine seismic survey, seismic energy sources are used to generate a seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) and recorded.
The appropriate seismic sources for generating the seismic signal in marine seismic surveys typically include a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield is typically an air gun or a spatially-distributed array of air guns.
The appropriate types of seismic sensors typically include particle velocity sensors (known in the art as geophones) and water pressure sensors (known in the art as hydrophones) mounted within a towed seismic streamer (also known as a seismic cable). Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays within the streamer.
After the reflected wave reaches the seismic sensors, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the sensors. The reflected wave continues to propagate and can be reflected upwardly again, by the water bottom or other subterranean formation interfaces. Reflected waves that reflect more than once are termed “multiples” and are typically treated as noise. A particular category of noise comprises multiples that reflect at least once from the water surface and are called surface-related multiples.
Three-dimensional surface-related multiple prediction (3D SRMP), a process to estimate surface-related multiples in seismic data, is a part of 3D surface-related multiple elimination (3D SRME), which strives to attenuate the surface-related multiples by a prediction-subtraction process. The surface-related multiples are first predicted from the seismic data and then the predicted multiples are subtracted from the seismic data to leave a noise-attenuated signal. A first step in this process comprises constructing a multiple contribution gather for a source-receiver trace, which involves the computation of the convolution of pairs of traces over a spatial area. A second step comprises constructing a predicted multiple trace which contains primarily multiple reflections, which involves stacking all the multiple contribution traces belonging to the source-receiver trace. A third step comprises subtracting the predicted multiple traces from the original seismic data.
Thus, the process of predicting 3D surface-related multiples from seismic data, for the combination of a specific source and a specific receiver location, consists of accumulating the results of convolving traces in pairs over a surface area of possible linkage locations. Typically, however, not all pairs of traces to be convolved at all linkage locations are always readily available from seismic data acquisition. This is due to the under-sampling of source and receiver positions, especially in the cross-line direction, that is inherent in marine seismic data acquisition using towed streamers. Furthermore, practical limitations in positioning and navigation systems, as well as wave currents that cause feathering of the streamers and source configuration, prohibit that sources and receivers are positioned exactly where they are desired.
Several methods exist that aim to regularize data in order to reconstruct missing data. These methods may be applied to generate data for 3-D surface-related multiple prediction. These methods known to the industry are based on NMO (Normal Moveout), full DMO (Dip Moveout)—Inverse DMO, AMO (Azimuth Moveout), or migration operators. These methods have in common that they aim to generate new data from the acquired data that are more similar to the desired traces.
Thus, a need exists for a method for data reconstruction to generate desired traces from the set of acquired traces, in order to achieve more accurate 3D surface-related multiple prediction.